MR. OWEN ON THE GENERATION OF THE MARSUPIAL ANIMALS. 347 



Mr. Morgan tried a similar experiment with a mammary foetus about the size of 

 a Norway Rat, which after two hours' separation from the nipple regained its hold, 

 and sustained no injury from the interruption of the supply of nourishment. The 

 evidence, therefore, which has been adduced establishes the fact that the mammary 

 foetus at a very early period is at least capable of sustaining a separation from the 

 nipple ; and although it may not at this stage of growth possess the power of regain- 

 ing its hold by its own unaided efforts, it is far from being the inert and formless 

 embryo which it has been described to be : it resembles, on the contrary, in its vital 

 powers, the new-born young of the smaller Mammalia rather than the uterine foetus 

 of a larger species when at a period of development at which this corresponds in size 

 to a new-born Kangaroo. 



By comparing the new-born Kangaroo with such a foetus, we find that although 

 in the Kangaroo the ordinary laws of development have been adhered to in the more 

 advanced condition of the anterior part of the body and corresponding extremities, 

 yet that the brain does not present so disproportionate a size ; and the same differ- 

 ence is observable in the uterine foetus of the Kangaroo (Plate VII. fig. 3.), even 

 when compared with the same-sized embryo of an animal of an inferior class 

 (Plate VII. fig. 4.). This difference, I apprehend, is owing to the rapidity with which 

 the heart and lungs acquire their adult structure in the Kangaroo, whereby the 

 passage of the purer and more nutritious blood through the foramen ovale and left 

 auricle to the primary branches of the aorta is arrested. The brain, however, of the 

 mammary foetus, though exhibiting a low degree of development, yet is of a firmer 

 texture than in a similarly sized foetus of a Sheep, and attains its ultimate propor- 

 tions by a more gradual process of growth. 



The brain and spinal chord (Plate VII. fig. 9 — 12.) were taken from a mammary 

 foetus of the Kangaroo, which measured one inch and a half in length, and which 

 was kindly presented to me by Mr. Allan Cunningham, of Kew. 



In this foetus I first observed the urinary bladder developed, and adhering by its 

 apex to the peritoneum exactly opposite that part of the abdominal integument where 

 a small linear ridge indicated the previous attachment of the umbilical appendage. 

 There were also minute but distinct traces of umbilical arteries running up the sides of 

 the bladder to this point of attachment. As the urinary bladder becomes afterwards 

 expanded, the peritoneum is gradually, as it were, drawn from this part of the abdo- 

 minal parietes, forming an anterior ligament of the bladder. In a mammary foetus 

 of the Kangaroo about a month older than the above, there was at the superior part 

 of this duplicature a small projecting point from the bladder, like the remains of a 

 urachus ; but the fundus, now developed considerably above this point, was covered 

 with a perfectly smooth layer of peritoneum, and it is this, I apprehend, which has 

 given rise to the belief that there was no trace of urachus or umbilical arteries in 

 the foetuses of the Marsupiata. In the Sloth, the Manis, and the Armadillo, the ura- 



